Volume holograms were invented by Yu Denisyuk in 1962 (see Yu.N. Denisyuk. xe2x80x9cPhotographic reconstruction of the optical properties of an object in its own scattered radiation field.xe2x80x9d Soviet Physics Dokl, 7 pp.17-21,1962 {hereinafter referred to as [Ref. 1]}) and initially used for image reconstruction with non-coherent radiation. However, in 1972 Kogelnik et al disclosed the use of volume holograms as reflective type narrow-band filter provided by the combination of a transmitting hologram and a mirror in U.S. Pat. No. 3,675,990 issued Jul. 11, 1972 {hereinafter referred to as [Ref. 2]} This patent can be considered as the first application of volume holograms as optical elements using their features of strong angular and spectral selectivity. Its combination of a transmitting hologram and a conventional mirror realized the creation of an efficient reflective filter by fabrication of volume grating with relatively small spatial frequency and having about 50% diffraction efficiency.
The further significant evolution of the approach described in Ref. [2] developed in the 1990s because of new requirements of the optical industry (spectral selection for image recognition and optical communications) and development of new photosensitive holographic materials. It was proposed in D. E. Keys et al, U.S. Pat. No. 4,965,152 issued Oct. 23, 1990 {hereinafter referred to as Ref. [3]} to produce notch filters for laser lines suppression based on reflective volume holograms recorded in photopolymers. This filter has shown attenuation of laser lines in the visible spectral region up to 1000 times. However, no data on spectral width and side lobes were published. Of course, the problem of the very soft and chemically active layer is a disadvantage of the discussed approach.
Creation of photo-refractive crystals (e.g. iron-doped lithium niobate) enabled a more advanced approach for diffractive optical elements see: G. A. Rakuljic et al, U.S. Pat. No. 5,440,669 issued Jul. 2, 1992 {hereinafter referred to as [Ref. 4]}; G. A. Rakuljic, V. Leyva. xe2x80x9cVolume holographic narrow-band optical filterxe2x80x9d. Optics Lett. 18, 459-461(1993) {hereinafter referred to as {Ref. 5]}; and, G. A. Rakuljic et al, U.S. Pat. No. 5,684,611 issued Nov. 4, 1997 {hereinafter referred to as [Ref. 6]}. Narrow-band spectral filters based on volume diffractive gratings recorded in photo-refractive crystals were created with spectral width in visible region down to 0.05 nm. These filters were proposed to serve in both planar and non-planar geometry for correlators and telescopes. This new and very promising approach had some disadvantages because of such features of photo-refractive crystals as opacity in the UV region, sensitivity to visible radiation after recording, and low thermal stability.
Thus, the further progress in diffractive elements developing was restrained by the lack of appropriate photosensitive materials. P. Hariharan in his recent book xe2x80x9cOptical Holography, Principles, Techniques, and Applications.xe2x80x9d Chapter 7 pp. 95-124 (Cambridge University Press, 1996) {hereinafter referred to as [Ref. 7]} on page 96 reports that the main photosensitive materials available for high efficiency hologram recordings are silver halide photographic emulsions, dichromated gelatin, photoresists, photopolymers, photothermoplastics, polymers with spectral hole-burning, and photo-refractive crystals. Each of these materials has their merits, but all have drawbacks. These organic materials (photographic emulsions, dichromated gelatin, and photopolymers) are sensitive to humidity. Moreover, they significantly shrink in the development process. Inorganic materials (photo-refractive crystals) have low resistance to elevated temperatures and produce additional patterns because of exposure to the beam diffracted from the recorded grating.
S. D. Stookey in an article xe2x80x9cPhotosensitive glass, (a new photographic medium)xe2x80x9d Industrial and Engineering Chem., 41, 856-861 (1949) {hereinafter referred to as [Ref. 8]} reported a two-step process (exposure and thermal development) to record a translucent image in glass because of light scattering caused by a difference between refractive indices of a precipitated crystalline phase and the glass matrix. According to this disclosure, the first step is the exposure of the glass sample to UV radiation, which produces ionization of a cerium ion. The electrons released from cerium are then trapped by a silver ion. As a result, silver is converted from a positive ion to a neutral atom. This stage corresponds to a latent image formation and no significant coloration occurs.
The next step is a thermal development. This development process includes two stages described in publications [9, 10]. The first stage involves the high diffusion rate silver atoms possess in silicate glasses. This diffusion leads to creation of tiny silver crystals at relatively low temperatures (450-500xc2x0 C.). A number of silver clusters arise in exposed regions of glass after aging at elevated temperatures. These silver particles serve as the nucleation centers for sodium and fluorine ion precipitation and cubic sodium fluoride crystal growth occurs at temperatures between 500xc2x0 C. and 550xc2x0 C. Further heat treatment leads to growth of numerous elongated pyramidal complexes of (Na,Agxe2x80x94F,Br) crystals on the surface of cubic NaF crystals. This mixture of crystals can produce opal coloration in the case of large crystal sizes or yellow coloration caused by colloidal silver precipitated on interfaces of dielectric crystals. This multi-stage photo-thermal process in photosensitive glass was proposed for decoration, color photography, sculpture, and even for holography see: S. D. Stookey, G. H. Beall, J. E. Pierson. xe2x80x9cFull-color photosensitive glassxe2x80x9d. J. Appl. Phys., 49, pp.5114-5123 (1978) {hereinafter referred to as [Ref. 9]; N. F. Borrelli, J. B. Chodak, D. A. Nolan, T. P. Seward. xe2x80x9cInterpretation of induced color in polychromatic glassesxe2x80x9d. J. Opt. Soc. Amer., 69 pp.1514-1519 (1979) {hereinafter referred to as [Ref. 10]}; and N. F. Borrelli et al, U.S. Pat. No. 4,514,053 issued Apr. 30, 1985 {hereinafter referred to as [Ref. 11]}. However, no evidences of any hologram recorded in these glasses are disclosed in these references [Refs. 9-11].
Several years later, the use of inorganic photosensitive glasses for phase hologram recording rather than as a photographic medium was reported in the literature. It was reported therein that Bragg gratings were obtained both in: lithium-aluminum-silicate (see V. A. Borgman, L. B. Glebov, N. V. Nikonorov, G. T. Petrovskii, V. V. Savvin, A. D. Tsvetkov xe2x80x9cPhoto-thermal refractive effect in silicate glasses.xe2x80x9d Sov. Phys. Dokl., 34, 1011-1013 (1989) {hereinafter referred to as [Ref. 12]}); and sodium-zinc-aluminum-silicate (see L. B. Glebov, N. V. Nikonorov, E. I. Panysheva, G. T. Petrovskii, V. V. Savvin, I. V. Tunianova, V. A. Tsekhomskii xe2x80x9cPolychromatic glassesxe2x80x94a new material for recording volume phase hologramsxe2x80x9d Sov. Phys. Dokl., 35, 878-880 (1990) {hereinafter referred to as [Ref. 13]}); and, (see L. B. Glebov, N. V. Nikonorov, E. I. Panysheva, G. T. Petrovskii, V. V. Savvin, I. V. Tunimanova, V. A. Tsekhomskii xe2x80x9cNew ways to use photosensitive glasses for recording volume phase holograms.xe2x80x9d Opt. Spectrosc., 73, 237-241 (1992) {hereinafter referred to as [Ref. 14]})glasses doped with silver and cerium, by exposure to UV radiation followed by thermal treatment. This phenomenon was named the xe2x80x9cphoto-thermo-refractivexe2x80x9d (PTR) process. Glasses, which possess such properties, were called xe2x80x9cphoto-thermo-refractivexe2x80x9d (PTR) glasses.
It was reported in Refs. [13] and [14] that a refractive index decrease of about 5xc3x9710xe2x88x924 occurs when dielectric crystals precipitated in glasses are exposed to radiation of a nitrogen laser at 337 nm. Conditions of glass exposure and development were found in those works to create Bragg gratings with relative diffraction efficiency up to 90%. These gratings were stable up to 400xc2x0 C. The photosensitivity was found in the range of several J/cm2 at a nitrogen laser wavelength (337 nm). Once developed, holograms in PTR glass were not destroyed by further exposure to visible light. Unfortunately, these materials reported in Ref. [12-14], do not met all requirements formulated in Hariharan, particularly absolute diffraction efficiency [6, Table 7.1 at page 96] because their property of excessive (strong) scattering of the radiation imposed on the hologram. This scattering results in low absolute diffraction efficiency of gratings in PTR glasses, which has been found not to exceed 45%.
The first object of the present invention is to provide diffractive gratings in glasses having the physical property of induced refractive index increment combined with low losses which enables high diffraction efficiency.
The second object of the present invention is to provide diffractive gratings in photo-thermo-refractive (PTR) glasses having the physical properties of high diffraction efficiency, low losses, high photosensitivity, high optical quality, high stability at elevated temperatures, humidity, and powerful optical illumination of volume for diffractive optical elements.
The third object of the present invention is to utilize high spectral and angular selectivity of volume diffractive gratings recorded in such homogeneous and transparent material as PTR glass for such diffractive optical elements as attenuators, beam splitters with arbitrary number of channels and splitting ratios, wavelength multiplexers and demultiplexers, angularly and spectrally controlled laser beam deflectors, longitudinal and transversal mode selectors in laser resonators, narrow-band spectral and angular selectors, spectral equalizers, and combinations of the mentioned elements in the same volume of glass.
The fourth object of the present invention is to provide illustrative devices based on the volume diffractive elements in PTR glasses.
Preferred embodiments of the invention are volume holographic optical elements of photo-thermal refractive (PTR) glass having an absolute diffraction efficiency of greater than approximately 50% and preferably having an absolute diffraction efficiency of at least approximately 90% and even up to approximately 96%.
Further objects and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments, which are disclosed in the following text and properties of which are illustrated in the accompanying drawings.